Output transformer fault or OK?

[DC Music]

This is an odd one guys. I have an output transformer installed in a AB763 Vibroverb combo, and the housing of the tranny has become magnetic. In other words, hover a screwdriver close to the end bell covers or anywhere on the transformer housing and the screwdriver will be magnetically drawn to the tranny. It's a reasonably strong pull. Is this a problem? I haven't noticed any issues with the performance of the amp. Maybe the amp breaks up a bit earlier than it used to.

Thanks in advance for any thoughts on this.

[J M Fahey]

How far is the transformer from the speaker magnet?
Is it a ceramic magnet?

[Leo_Gnardo]

Are you sure it's the transformer? With all that AC flux passing thru, I'd expect it pretty well demagnetizes itself every time it's used. OTOH practically all my hand tools develop magnetism, sometimes it's a real pain in the you know what. For those I use an Annis Han-D-Mag (heavy duty tape machine demagnetizer) or the butt end of an old fashioned Weller soldering gun to demagnetize. And as Juan mentions, in the presence of an existing magnetic field (speaker magnet) all other magnetizable items also develop a field. I've never seen it be a problem as far as amp operation is concerned.

[Enzo]

Contrasting my preferences: I keep a few speaker magnets (taken from the baskets) stuck on my tool box, so I can KEEP my screwdrivers magnetized. I rely on them to retain the screws on the tip while reaching. If I am working near the speaker, say unscrewing the reverb pan, the speaker magnet will grab the driver whether it is magnetized or not.

[DC Music]

How far is the transformer from the speaker magnet?
Is it a ceramic magnet?

Yep, ceramic magnet JBL E130 but the tranny will do this even when it's not installed in the amp.

[DC Music]

Are you sure it's the transformer? With all that AC flux passing thru, I'd expect it pretty well demagnetizes itself every time it's used. OTOH practically all my hand tools develop magnetism, sometimes it's a real pain in the you know what. For those I use an Annis Han-D-Mag (heavy duty tape machine demagnetizer) or the butt end of an old fashioned Weller soldering gun to demagnetize. And as Juan mentions, in the presence of an existing magnetic field (speaker magnet) all other magnetizable items also develop a field. I've never seen it be a problem as far as amp operation is concerned.

Thanks for your input Leo. Incidentally, like Enzo above, I actually prefer to have some magnetized screwdrivers. Makes certain little jobs way easier.

[Jazz P Bass]

As far as I know, a DC voltage passing through the winding will magnetize the core itself.

[J M Fahey]

True, but the EI magnetic circuit will drive *that* magnetization in a closed loop, nothing "should" be perceived outside the core.

Yes, a little escapes, that's why side by side transformers are slightly coupled (enough to annoy, but no *real* power is transmitted) but not enough, even if DC present, to attract iron parts.

If it weren't so, a choke woukd be a nightmare ... and we know they are not.

Commercial solenoids are made with a coil around an E core .... with no I present, so they break that tightly closed path I mentioned and it can go outside to do some useful work ... but it's not the case either in a choke or an OT.

I guess the core just got somewhat magnetized thanks to the monster 220mm JBL magnet, the largest in the Industry (EV uses 190mm).

And not in the close path direction which is normal in a transformer but in a straight outside path.

Too busy to draw the "arrows" which would make this clearer so you'll have to imagine them

[Enzo]

And for critical applications, they do make non-magnetic screwdrivers. I think they are mainly bronze or something, though I also have seen titanium.

Watchmakers and sensitive instrument makers don't want to magnetize anything, for example.

[mozz]

Just deguass it.

[DC Music]

Just deguass it.

This was my original intention but I wanted to check if the behaviour is representative of some kind of problem first.

[J M Fahey]

No, no problem at all.